Only thing that I know of is that there is a known issue that can occur if the body is empty. Don't know if that's the problem here or not.
The send mail scripts bring the Body and the Subject of the message over just fine. It just doesn't bring the TO email over.
Keep in mind that you can also report this as a possible bug over in Report an Issue.
If you select Send Mail from the File Menu and manually send an email to yourself, does it work? Can you refer to a field from this manual email's dialog and get the TO box to populate?
I had the user try to send email following th einstructions you provided and it did not work.
I did however set up another machine with the same setup and the process does work. So I am going to try to uninstall and reinstall the programs and see if that resolves the issue.